The destructive hydrogenation or hydrocracking of hydrocarbons is well-known. In these processes, aliphatic hydrocarbons undergo cracking to produce lower hydrocarbons such as propane, methane, and, especially, ethane. Alkylated aromatic hydrocarbons present undergo dealkylation to at least a minor extent. Non-alkylated aromatic compounds are generally unaffected to any appreciable extent, except under the most severe conditions.
In view of the foregoing, the light hydrocarbon gases of one to four carbon atoms have generally been synthesized as a by-product of such hydrogenation of hydrocracking of aliphatic hydrocarbons. Representative processes involving, for example, Naphtha are described in British Patent Specification Nos. 1,265,415 and 1,333,776. Recommended conditions for these processes include a high hydrocarbon to hydrogen ratio (i.e., the ratio of hydrogen fed to the reactor zone to that which is stoichiometrically required to convert all feedstock carbon to methane) and temperatures of about 1100.degree.-1400.degree. F. These conditions are designed to maximize production of ethane, which may then be converted to ethylene.
Hydrogasification of predominantly aliphatic liquid feedstock to produce ethane for subsequent steam cracking results in increased yields of ethylene, (as compared to directly steam cracking the feedstock), but at the expense of other valuable cracking co-products, such as propylene and butadiene. Although petroleum distillate, crude petroleum and heavy oils are claimed to be suitable feedstocks for the process described in British Patent Specification No. 1,265,415, that process is, in practice, limited to naphtha feedstock. With heavier feedstocks, coking is increased beyond that tolerable of many reactor types. While high yields of ethylene can be obtained by the hydrogasification of naphtha followed by steam cracking of the ethane product, naphtha can be directly steam cracked to yield comparable total C.sub.2 -C.sub.4 petrochemical products (ethylene, propylene and butadiene).
A method of processing these heavy aliphatic hydrocarbon feedstocks which has been the subject of numerous patents, contemplates the use of particulate catalyst beds, referred to in the art as "ebullated beds". Examples of such patents are U.S. Pat. Nos. 3,630,887; 3,248,319; 3,363,024; 3,412,010; 3,888,761; 3,576,899; 4,065,514; and 3,385,782. Use of these catalyst particles, according to U.S. Pat. No. 3,309,305, is desirable in order to accomplish effective contact between fluids and particles.
The prior art, such as U.S. Pat. No. 3,619,411 issued to Shell Oil Company, teaches that effective hydrogenation of heavy high-boiling hydrocarbons requires the presence of a catalyst in order to prevent unsaturated fragments from condensing to form coke. The disadvantages of heavy feedstock processes include the identification and addition of a catalyst that will survive the often severe and highly coking reaction conditions. The need for removal of the catalyst from the effluent stream is also a drawback.
Thus, the light paraffinic hydrocarbon gases, i.e., methane through butane, have generally been synthesized at relatively low yields through the catalytic cracking and hydrogenerating of aliphatic hydrocarbons. Because the primary processing value of certain of these gases, specifically ethane, is for the production of ethylene, use of predominantly aliphatic hydrocarbon feedstocks has until now been necessary in order to maximize ethylene yields. This maximization occurs at the expense of valuable co-products of gas oil steam cracking such as propylene and butadiene.
Because of the increasing demand for light hydrocarbon gases, it would be valuable to have a process for producing light gases in higher yields than have generally been obtainable. Light hydrocarbon gases of one to four carbon atoms are generally produced from petroleum at the expense of gasoline. It would be particularly valuable to have a method of processing high boiling feeds, particularly aromatic feedstocks, directly to light hydrocarbon gases as a principal reaction product. These are more difficult to refine and less valuable for gasoline production.
It is also desirable to develop a process that would not only provide high yields of light aliphatic gases, but which would neither require the presence of a catalyst nor consume valuable liquid steam cracking feedstocks, such as naphtha and petroleum gas oils. Such a process could only evolve where the amount of coking was significantly reduced and where ethylene via ethane could be derived from an otherwise low value feedstock.
In an attempt to arrive at such a process, U.S. Pat. No. 4,115,467 issued to Fowler teaches the production of a C.sub.2 hydrocarbon from higher hydrocarbon feeds by hydrogenation in a fluidized bed. This bed may, but need not, contain catalyst. The temperature is maintained above the threshold temperature for the reaction by supplying hot combustion gases to the hydrogenation zone, making the subject process less energy efficient.
In an unrelated process, U.S. Pat. No. 4,139,452 describes the hydrogenation of coal liquids and fluid catalytic cracker liquids. This process results in some by-product ethane; however, it is primarily directed to the production of benzene.